Enameling process and article produced thereby



United States Patent ENAMELING PROCESS AND ARTICLE PRODUCED THEREBYWaldo W. Higgins, Kankakee, Ill., assignor to A. 0. Smith Corporation,Milwaukee, Wis., a corporation of New York No Drawing. Originalapplication September 22, 1943, Serial No. 503,442. Divided and thisapplication May 6, 1949, Serial No. 91,876

3 Claims. (Cl. 29195) This invention relates to enameling processes andarticles produced thereby. It has been applied to the ceramic enamelingof steel base ordinarily of rolled sheet stock, and is applicable to thecoating of steel with many types of applied coatings.

In ceramic enameling, the coating material is usually applied to thesurface of the steel either by dipping or spraying, and after drying itis fired at a temperature sufficiently high to fuse the material andproduce a glossy impervious coating bonded throughout to the steel base.Depending upon the enamel composition, the firing temperature may rangefrom about 1300 F. for soft enamels to as high as 1700 F. for hardenamels. In order to produce the desired bond, it is customary to employa ground coat material, usually containing cobalt oxide, as the firstcoat, and to apply one or more subsequent cover coats of a materialusually not adaptable for ground coats. Each coat is separately fired.

It has been observed that in the firing of the enamel ground coat a veryconsiderable amount of boiling occurs in which minute bubbles rise tothe surface from the interface between the steel base and the coating,known as primary boiling. Copper head defects result from excessiveprimary boiling when the larger bubbles break and expose the steel base.

It has been assumed in the past that the bubbles result from gasformation from the reaction of the various constituents of the coatingand also from reaction between the coating and the base. Only recentlyhas it been suggested that some or all of the gas forming the bubblesmight result from the giving off of gas from the steel base, possiblyhydrogen gas released from the steel and driven off as the steel heatsup to the firing temperature.

Many defects in enamel can be traced to hydrogen resulting from anexcess of this gas in the steel. instance, it has been thought that fishscaling of the enamel is the result of hydrogen gas pressure. Tests haveshown that when an acid such as dilute sulphuric acid is allowed to eatthe back of an enameled sheet steel base in a restricted area, theenamel coating on the front begins to fish scale severely. This isbelieved to be due to the nascent hydrogen given off as a result of thereaction of the acid with the steel, the hydrogen diffusing through thesteel to the opposite side where it builds up suificient pressure anddestroys the enamel bond. See Journal of the American Ceramic Society,vol. 23, No. 7, pages 187 to 221 and vol. 24, No. 12, pages 383 to 392.

In applying cover coats a phenomenon known as reboiling occurs, in whichminute bubbles rise up from the ground coat steel interface causingrupture of the cover coat and forming black specks and the like. Thisphenomenon has been attributed to the presence of hydrogen in the steel.See Zapfle and Sims on Relation of defects in enamel coatings tohydrogen in steel, Journal of the American Ceramic Society, vol 23, No.7, pages 187 to 221 (1940). The same is true with For . ing defectsreferred to.

ice

respect to defective bonding of enamels. See Journal of the AmericanCeramic Society, vol. 26, No. 5, May 1, 1943, pages 151 to 159.

The present inventor has discovered that where excessive primary boilingoccurs in enameling, there is a strong likelihood that reboiling willoccur and that the enamel will fish scale. From this discovery the causeof primary boiling would seem to be hydrogen in the steel, although noone heretofore has known or suggested this fact.

Another defect occurring in ground coats results when an excess ofmoisture or water vapor is present in the furnace atmosphere duringfiring. The defect embodies a rough orange peel type of surface known aswater vapor condition. An examination of the coat shows that close tothe interface between the enamel and steel there is a layer of largebubbles, there being a bubble beneath each hump in the surface. Thisfairly continuous layer of bubbles gives a zone of weakness in theenamel coating, allowing the enamel to spall. tion is very likely tooccur in commercial enameling operations where the dew point of thefurnace atmosphere gets above F. and where relatively thin sheet steelpieces are being coated simultaneously on both sides with enamel.

These various defects are serious in every enameling plant and result ina balancing of enameling composition, the processing and equipmentdesign, and the steel with cut-and-try methods until a particulartechnique is developed for a given product in the particular plant,which is adhered to closely in order to avoid trouble. This makes theinterchange and reconciliation of detail information in the enamelingindustry difiicult.

The present invention is based upon the conception of the presentinventor that most all of these defects result from hydrogen in thesteel, and that if the steel could be made to resist the diffusion,absorption or release of hydrogen, the various techniques andcompositions involved in enameling would no longer be critical in theproduction of perfect coats of enamel. The invention is further basedupon the discovery that certain alloy constituents can be added to steelor to its surface to have What is thought to be one or more of thefollowing effects: (1) preventing or retarding the difiusion of nascenthydrogen through the steel; (2) preventing or reducing the absorption ofhydrogen by the steel; and (3) preventing the release of hydrogen fromthe steel in sufiicient quantity to injure the enamel. Takenindividually and collectively the above enumerated effects may be termeda stabilization of hydrogen and the constituents having such effects maybe called hydrogen stabilizers.

The principal object of the present invention is to prevent injury tocoatings by evolution of hydrogen from the base metal.

Another object of the invention is to provide an enamel coated steelarticle in which the enamel is free from the defects caused by primaryboiling and also from fish scaling, reboiling and water vapor condition.

Another object is to provide a method of making ceramic enameled steelarticles free from defects in the enamel without regard to carefultechnique, enamel composition and special firing atmospheres.

In support of these objects the invention results fromthe discovery thatthe addition of certain alloy constituents to the steel or its surfaceeliminates the enamel- Chromium and titanium additions to the steel havethe desired effect. These may be added either singly or in combinationto the steel.

The process, in its most comprehensive phase, involves the prevention ofhydrogen phenomena injurious to coatings by adding to the steel, eitherin its manufacture Water vapor condi-- or by subsequent impregnation anddiifusion, a metal producing one or more of the eifects enumeratedabove.

The process as applied to ceramic enameling involves the applyingandfiringof the enamel in the presence of a metal such. as chromium,titanium, 'or the like, alloyed with the steel base insuflicient-quantity to effect the desired results with" respect tohydrogen phenomena, and insufficient-to embrittle or otherwise changethe physical properties of the steel in a manner making it unsuitablefor use for sheet enameling stock.

A composition of steel found to give excellent results contained about.50%' titanium with .07% carbon and .39% manganese. Good results havebeen obtained with as low as 30% of titanium, and tests show that thereshould be a-considerable excess of titanium above the amount required tokill the'steel. The upper limit of titanium content is not known andpossibly should depend uponother factors such as cost, otherconstituents and the physical characteristics desired.

In the case of chromium, excellent results have been obtained with theordinary 4 to 6% chromium steel.

As low as 1% of chromium-in the steel gives very superior results overpresent-day enameling stocks. However, chromium has a tendency to makesteel ditficult to fabricate, and-the amount that can be employeddepends upon the fabrication requirements of the article and the costinvolved. It should be kept as low as possible consistent with goodenameling results and with the requirements as to physical properties ofthe steel. In general a 1% chromium steel may be substituted for mostenamelingstocks.

'The steel should be reasonably lowin alloying constituents, preferablyless than since the addition of many alloy ingredients lowers thetransformation point to such an extent as to cause crazing. This isparticularly true of vanadium, molybdenum, silicon,'manganese andnickel. v

. Samples of steel of the compositions referred to can be ceramicenameled without danger of producing primary boiling, reboiling, fishscaling, or water vapor condition.

Such samples of sheet steel containing titanium in the amountsindicated, "coated on one side with ceramic enamel-andsubjected to thesulphuric acid test on the other side, proved'to retain the enamel coatwithout fish scaling or other defect." The firing of the articles incarbon stabilization was important, but this is not. the property ofchromium, and molybdenum which is considered a good carbon stabilizerdoes not produce the desired results where added in small amounts.

Cover coats heretofore considered unsuitable for direct application tosteel have been applied to steels contain: ing chromium or titanium inthe amounts specified with excellent results. White enamel coats whichheretofore had to be applied over a dark ground coat have been applieddirectly to steels containingchromium or titanium, thereby accomplishingone of the dreams of every commercial enameler.

Similarly, acid proof enamels can be applied directly to the steelwithout requiring ground coats which tend to contaminate the enamels andreduce their acid resistance.

The invention is therefore applicable to either single or multiple coatenameling and eliminates the difliculties of contamination frompresent-day ground coats.

The importance of the present invention arises from the fact that manydefects in ceramic enamels can-be absolutely eliminated by its practice.and lacquers, as. well as electro-plated coatings, when applied to steelare more sensitive to hydrogen evolution destroying their bond, theinvention is applicable to protect such coatings from disruption inservices where heating and cooling or other treatment has a tendenc tocause undesirable hydrogen phenomena.

This application constitutes a continuation of the copendingapplication'filed by applicant on September 22, 1943, Serial No.503,442, for the same invention, and now abandoned.

Various embodiments of the invention may be employed within the scope ofthe accompanying claims.

I claim:

1. The process of producing a coated steel article, comprising selectinga killed steel base for the article, im-

pregnating the steel surface of the article with from 1% atmospheres ofhigh water vapor content does not seem to injure the enamel or toproduce the rough orange peel like surface so noticeable in such firingof enamel heretofore. The same was true with respect to chromium steelsamples.

The impregnation of the surface of steel with corresponding amounts'ofeither chromium or titanium gives the same results as the adding of suchingredients to the steel during its manufacture. Samples'of low carbon,low alloy enameling stock which were packed in either ferrochrome orferro-titanium and heated to a temperature and for a time suflicient tocause diffusion of a substantial or appreciable amount of the materialinto the steel surface for a sufiicient depth, had the desiredcharacteristics with respect to hydrogen phenomena.- For example, animpregnation to a depth of about three-thousandths of an inch gave veryexcellent results.

This result is interesting in view of the fact that attempts to coat thesurface of the steel with ferrochromium and ferro-titanium prior toenameling without impregnation proved futile and did not give thedesired result.

It is also interesting to note that metals ordinarily employed forkilling steel, such as silicon and aluminum, do not produce the results.At one time it was thought that to 6% chromium to prevent hydrogenphenomena, and thereafter applying to said surface a coating which issusceptible to damage by hydrogen phenomena, where: by said coating isbonded to said surface and is free from defects due to hydrogenphenomena.

2. The process of producing a coated steel, article, comprisingselecting a killed steel base for the article, impregnating the steelsurface of the article with an appreciable quantity of titanium to.prevent hydrogen phenomena, and thereafter applying to said' surface acoating which is susceptible to damage by hydrogen phenomena, wherebysaid coating is bonded tosaid surface and is free from defects due tohydrogen phenomena.-

3. An article of manufacture comprising a low alloy killed steel basehaving a total alloy content not in ex-- cess of 5% and which alloycontent consists principally of a hydrogen phenomena inhibitor selectedfrom the. groupconsisting of chromium and titanium and alloyed with thesurface thereof for a substantial depth, and a:

ceramic enamel coating fired on the. surface thereof and; free fromdefects caused by hydrogen phenomena.

References Citedin the file of this patent UNITED STATES PATENTS1,996,568. Butts Apr.'2, 1935 2,059,893 Naumann Nov. 3, 1936 2,279,935Belding Apr. 14, 1942 2,495,836 Comstock Jan. 31, 1950 OTHER REFERENCESTitanium and Its Use in Steel, Titanium Alloy Manufacturing Co., 1940.

Since many paints

1. THE PROCESS OF PRODUCING A COATED STEEL ARTICLE, COMPRISING SELECTINGA KILLED STEEL BASE FOR THE ARTICLE, IMPREGNATING THE STEEL SURFACE OFTHE ARTICLE WITH FROM 1% TO 6% CHROMIUM TO PREVENT HYDROGEN PHENOMENA,AND THEREAFTER APPLYING TO SAID SURFACE A COATING WHICH IS SUSCEPTIBLETO DAMAGE BY HYDROGEN PHENOMENA, WHEREBY SAID COATING IS BONDED TO SAIDSURFACE AND IS FREE FROM DEFECTS DUE TO HYDROGEN PHENOMENA.